1. Field of the Invention
The present invention relates to a membrane electrode assembly (also referred to as an MEA), a manufacturing method thereof and a fuel cell using the MEA. In particular, the present invention relates to an MEA which has two first electrode catalyst layers as stacked layers different in polymer electrolyte content ratio, and a manufacturing method thereof and a fuel cell (especially, PEFC: polymer electrolyte fuel cell or PEMFC: proton exchange membrane fuel cell) using the MEA.
2. Description of the Related Art
Fuel cells are power generation systems which produce electric power along with heat. A fuel gas including hydrogen and an oxidant gas including oxygen reacts together at electrodes containing catalyst so that the reverse reaction of water electrolysis takes place in a fuel cell. Fuel cells are attracting attention as a clean energy source of the future since they have advantages such as a small impact on the environment and a low level of noise production relative to conventional power generation systems. Fuel cells are divided into several types according to the employed ion conductor. A fuel cell which uses an ion-conductive polymer membrane is called a PEFC or PEMFC.
Among various fuel cells, a PEFC (or PEMFC), which can be used at around room temperature, is considered as a promising fuel cell for use in a vehicle and a household stationary power supply etc. and is being developed widely in recent years. A joint unit which has a pair of electrode catalyst layers on both sides of a polymer electrolyte membrane and which is called a membrane electrode assembly (MEA) is arranged between a pair of separators, on which either a gas flow path for supplying a fuel gas including hydrogen to one of the electrodes or a gas flow path for supplying an oxidant gas including oxygen to the other electrode is formed, in the PEFC (PEMFC). The electrode for supplying a fuel gas is called a fuel electrode or cathode (electrode), whereas the electrode for supplying an oxidant gas is called an air electrode or anode (electrode). Each of the electrodes includes an electrode catalyst layer, which has stacked polymer electrolytes with carbon particles on which a catalyst such as a noble metal of platinum group is loaded, and a gas diffusion layer which has gas permeability and electron conductivity.
Conventionally, various manufacturing methods of membrane electrode assembly have been studied to improve the fuel cell performance. Examples of the manufacturing method of membrane electrode assembly include a method in which a catalyst layer is formed as an electrode by coating a coating solution containing a catalyst onto the ion-exchange membrane and the electrode and the ion-exchange membrane are joined by a heat treatment such as hot press to prepare the membrane electrode assembly, a method in which a catalyst layer is formed on a substrate film that is prepared independently of an ion-exchange membrane and the ion-exchange membrane is stacked on the catalyst layer to transfer the catalyst layer onto the ion-exchange membrane by the hot press, a method in which an electrode sheet in which the catalyst layer is formed is prepared on a gas diffusion layer to join the electrode sheet to the ion-exchange membrane, and a method in which two sets of half cells in which the catalyst layer is formed on the ion-exchange membrane are prepared, surfaces of the ion-exchange membrane sides are pressure-bonded while faced to each other, thereby preparing the membrane electrode assembly.
However, because the membrane electrode assembly produced by the conventional methods is prepared by the thermal pressure bonding such as the hot press when joining the ion-exchange membrane and the electrode catalyst layer, the thermal pressure bonding process becomes a bottleneck, and a tact time is lengthened, which unfortunately degrades production efficiency.    <Patent document 1> JP-A-2003-197218    <Patent document 2> JP-A-2005-294123    <Patent document 3> JP-A-2005-108770
On the other hand, in a sequentially stacking type MEA (Membrane Electrode Assembly) in which a first electrode catalyst layer is prepared on a substrate, a polymer electrolyte layer is prepared, and finally a second electrode catalyst layer is prepared, the producing cost is reduced because the tact time is shortened to enhance the production efficiency. However, a polymer electrolyte invades into the porous first electrode catalyst layer to reduce a void, and therefore a MEA gas diffusion property is degraded and flooding is easily generated.
As to the sequentially stacking type MEA technique, for example, JP-A-2003-197218, JP-A-2005-294123, and JP-A-2005-108770 disclose the method in which the first electrode catalyst layer is prepared on the substrate, the polymer electrolyte layer is prepared, and finally the second electrode catalyst layer is prepared. Specifically, JP-A-2003-197218 and JP-A-2005-294123 disclose that a drying process is required in order to prevent the polymer electrolyte from invading in and mixing with the first electrode catalyst layer. JP-A-2005-108770 discloses that a drying speed is defined to prevent the polymer electrolyte from invading in the first electrode catalyst layer. However, in the techniques disclosed in JP-A-2003-197218, JP-A-2005-294123, and JP-A-2005-108770, it is difficult to prevent the polymer electrolyte from invading in the first electrode catalyst layer, the void is reduced, and therefore the MEA gas diffusion property is degraded and the flooding is easily generated.